This invention generally relates to autostereoscopic display systems for viewing electronically generated images and more particularly relates to an apparatus and method for generating left- and right-eye images using a resonant fiber-optic member to form an image, with a monocentric arrangement of optical components including a retro-reflective surface to provide a very wide field of view and large exit pupils.
The potential value of autostereoscopic display systems is widely appreciated particularly in entertainment and simulation fields. Autostereoscopic display systems include xe2x80x9cimmersionxe2x80x9d systems, intended to provide a realistic viewing experience for an observer by visually surrounding the observer with a three-dimensional (3-D) image having a very wide field of view. As differentiated from the larger group of stereoscopic displays that include it, the autostereoscopic display is characterized by the absence of any requirement for a wearable item of any type, such as goggles, headgear, or special glasses, for example. That is, an autostereoscopic display attempts to provide xe2x80x9cnaturalxe2x80x9d viewing conditions for an observer.
In an article in SID 99 Digest, xe2x80x9cAutostereoscopic Properties of Spherical Panoramic Virtual Displaysxe2x80x9d, G. J. Kintz discloses one approach to providing autostereoscopic display with a wide field of view. Using the Kintz approach, no glasses or headgear are required. However, the observer""s head must be positioned within a rapidly rotating spherical shell having arrays of light emitting diodes (LEDs), imaged by a monocentric mirror, to form a collimated virtual image. While the Kintz design provides one solution for a truly autostereoscopic system having a wide field of view, this design has considerable drawbacks. Among the disadvantages of the Kintz design is the requirement that the observer""s head be in close proximity to a rapidly spinning surface. Such an approach requires measures to minimize the likelihood of accident and injury from contact with components on the spinning surface. Even with protective shielding, proximity to a rapidly moving surface could, at the least, cause the observer some apprehension. In addition, use of such a system imposes considerable constraints on head movement.
One class of autostereoscopic systems that operates by imaging the exit pupils of a pair of projectors onto the eyes of an observer is as outlined in an article by S. A. Benton, T. E. Slowe, A. B. Kropp, and S. L. Smith (xe2x80x9cMicropolarizer-based multiple-viewer autostereoscopic displayxe2x80x9d, in Stereoscopic Displays and Virtual Reality Systems VI, SPIE, January, 1999). Pupil imaging, as outlined by Benton in the above-mentioned article, can be implemented using large lenses or mirrors. An observer whose eyes are coincident with the imaged pupils can view a stereoscopic scene without crosstalk, without wearing eyewear of any kind.
It can be readily appreciated that the value and realistic quality of the viewing experience provided by an autostereoscopic display system using pupil imaging is enhanced by presenting the 3-D image with a wide field of view and large exit pupil. Such a system is most effective for immersive viewing functions if it allows an observer to be comfortably seated, without constraining head movement to within a tight tolerance and without requiring the observer to wear goggles or other devices. For fully satisfactory 3-D viewing, such a system should provide separate, high-resolution images to right and left eyes. It can also be readily appreciated that such a system is most favorably designed for compactness, to create an illusion of depth and width of field, while occupying as little actual floor space and volume as is possible. For the most realistic viewing experience, the observer should be presented with a virtual image, disposed to appear a large distance away.
An example of a conventional autostereoscopic display unit is disclosed in U.S. Pat. No. 5,671,992 (Richards), at which a seated observer experiences apparent 3-D visual effects created using images generated from separate projectors, one for each eye, and directed to the observer using an imaging system comprising a number of mirrors and a retro-reflective surface. The apparatus disclosed in U.S. Pat. No. 5,671,992 does not provide a wide-field of view, however, which would be advantageous for immersive autostereoscopic display, as noted above.
Other uses of retro-reflective surfaces for autostereoscopic imaging are disclosed in U.S. Pat. Nos. 5,572,363 and 5,629,806 (both to Fergason). The Fergason patents disclose in-line and folded optical path arrangements using a retro-reflective surface and beamsplitter that cooperate to enlarge an image projected from a relatively small image source in order to provide, at a viewing pupil, a real image formed on the retro-reflective surface of the device. In the approach disclosed in the Fergason patents, the viewing pupil is a pseudo-image of the exit pupil of the projection lens. A large viewing pupil with wide field of view are preferred for ease of viewing; however, in order to form a large viewing pupil using conventional wide-field lenses, the apparatus disclosed in U.S. Pat. Nos. 5,572,363 or 5,629,806 requires a low-gain retro-reflective screen. But such low-gain retro-reflective screens have disadvantages including reduced brightness and, due to gain profile characteristics, increased crosstalk between left- and right-eye images. Constrained by the need to provide a large viewing pupil given a small projector lens exit pupil, then, devices using low-gain retro-reflective surfaces tend to compromise on image quality. As was a drawback of the apparatus using a retro-reflective surface disclosed in U.S. Pat. No. 5,671,992, the apparatus disclosed in U.S. Pat. Nos. 5,572,363 and 5,629,806 do not provide wide field of view.
Conventional solutions for stereoscopic imaging have addressed some of the challenges for inexpensively providing wide field of view with high brightness, but there is room for improvement. For example, some early stereoscopic systems employed special headwear, goggles, or eyeglasses to provide the 3-D viewing experience. As just one example of such a system, U.S. Pat. No. 6,034,717 (Dentinger et al.) discloses a projection display system requiring an observer to wear a set of passive polarizing glasses in order to selectively direct the appropriate image to each eye for creating a 3-D effect.
Certainly, there are some situations for which headgear of some kind can be considered appropriate for stereoscopic viewing, such as with specific simulation applications. For such an application, U.S. Pat. No. 5,572,229 (Fisher) discloses a projection display headgear that provides stereoscopic viewing with a wide field of view. However, where possible, there are advantages to providing autostereoscopic viewing, in which an observer is not required to wear any type of device, as was disclosed in the device of U.S. Pat. No. 5,671,992. It would also be advantageous to allow some degree of freedom for head movement. In contrast, U.S. Pat. No. 5,908,300 (Walker et al.) discloses a hang-gliding simulation system in which an observer""s head is maintained in a fixed position. While such a solution may be tolerable in the limited simulation environment disclosed in the Walker et al. patent, and may simplify the overall optical design of an apparatus, constraint of head movement would be a disadvantage in an immersion system. Notably, the system disclosed in the Walker et al. patent employs a narrow viewing aperture, effectively limiting the field of view. Complex, conventional projection lenses, disposed in an off-axis orientation, are employed in the device disclosed in U.S. Pat. No. 5,908,300, with scaling used to obtain the desired exit pupil size.
A number of systems have been developed to provide stereoscopic effects by presenting to the observer the combined image, through a beamsplitter, of two screens at two different distances from the observer, thereby creating the illusion of stereoscopic imaging, as is disclosed in U.S. Pat. No. 5,255,028 (Biles). However, this type of system is limited to small viewing angles and is, therefore, not suitable for providing an immersive viewing experience.
From an optical perspective, it can be seen that there would be advantages to autostereoscopic design using pupil imaging. A system designed for pupil imaging must provide separate images to the left and right pupils correspondingly and provide the most natural viewing conditions, eliminating any need for goggles or special headgear. In addition, it would be advantageous for such a system to provide the largest possible viewing pupils to the observer, so as to allow some freedom of movement, to maintain the necessary image brightness, and to provide an ultra-wide field of view. It is recognized in the optical arts that while an ideal autostereoscopic imaging system must meet these requirements in order to provide a more fully satisfactory and realistic viewing experience, satisfying all of these requirements can be difficult to achieve. In addition, such a system must provide sufficient resolution for realistic imaging, with acceptable contrast. Moreover, physical constraints for small system footprint and dimensional constraints for acceptable interocular separation must be considered, so that separate images directed to each eye can be advantageously spaced and correctly separated for viewing. It is instructive to note that interocular distance constraints limit the ability to achieve larger pupil diameter at a given ultrawide field by simply scaling the projection lens.
Monocentric imaging systems have been shown to provide significant advantages for high-resolution imaging of flat objects, such as is disclosed in U.S. Pat. No. 3,748,015 (Offner), which teaches an arrangement of spherical mirrors arranged with coincident centers of curvature in an imaging system designed for unit magnification. The monocentric arrangement disclosed in the Offner patent minimizes a number of types of image aberration and is conceptually straightforward, allowing a simplified optical design for high-resolution catoptric imaging systems. A monocentric arrangement of mirrors and lenses is also known to provide advantages for telescopic systems having wide field of view, as is disclosed in U.S. Pat. No. 4,331,390 (Shafer). However, while the advantages of monocentric design for overall simplicity and for minimizing distortion and optical aberrations can be appreciated, such a design concept can be difficult to implement in an immersion system requiring wide field of view and large exit pupil with a reasonably small overall footprint. Moreover, a fully monocentric design would not meet the requirement for full stereoscopic imaging, since an imaging system providing stereoscopic effects must present separate images for left and right pupils.
As is disclosed in U.S. Pat. No. 5,908,300, conventional wide-field projection lenses can be employed as projection lenses in a pupil-imaging autostereoscopic display. However, there are a number of disadvantages with conventional approaches. As was noted earlier with respect to U.S. Pat. Nos. 5,572,363 and 5,629,806, the relatively small exit pupil size of conventional projection lenses can be a limitation with negative impact on the design and performance of autostereoscopic display systems using retro-reflective surfaces. Wide-angle lens systems, capable of angular fields such as would be needed for effective immersion viewing, would be very complex and costly. Typical wide angle lenses for large-format cameras, such as the Biogon(trademark) lens manufactured by Carl-Zeiss-Stiftung in Jena, Germany for example, are capable of 75-degree angular fields. The Biogon lens consists of seven component lenses and is more than 80 mm in diameter, while only providing a pupil size of 10 mm. For larger pupil size, the lens needs to be scaled in size, however, the large diameter of such a lens body presents a significant design difficulty for an autostereoscopic immersion system, relative to the interocular distance at the viewing position. Costly cutting of lenses so that right- and left-eye assemblies could be disposed side-by-side, thereby achieving a pair of lens pupils spaced consistently with human interocular separation, presents difficult manufacturing problems. Interocular distance limitations constrain the spatial positioning of projection apparatus for each eye and preclude scaling of pupil size by simple scaling of the lens. Moreover, an effective immersion system most advantageously allows a very wide field of view, preferably well in excess of 90 degrees, and would provide large exit pupil diameters, preferably larger than 20 mm.
As an alternative for large field of view applications, ball lenses have been employed for specialized optical functions, particularly miniaturized ball lenses for use in fiber optics coupling and transmission applications, such as is disclosed in U.S. Pat. No. 5,940,564 (Jewell) which discloses advantageous use of a miniature ball lens within a coupling device. On a larger scale, ball lenses can be utilized within an astronomical tracking device, as is disclosed in U.S. Pat. No. 5,206,499 (Mantravadi et al.) In the Mantravadi et al. patent, the ball lens is employed because it allows a wide field of view, greater than 60 degrees, with minimal off-axis aberrations or distortions. In particular, the absence of a unique optical axis is used advantageously, so that every principal ray that passes through the ball lens can be considered to define its own optical axis. Because of its low illumination falloff relative to angular changes of incident light, a single ball lens is favorably used to direct light from space to a plurality of sensors in this application. Notably, photosensors at the output of the ball lens are disposed along a curved focal plane.
The benefits of a spherical or ball lens for wide angle imaging are also utilized in an apparatus for determining space-craft attitude, as is disclosed in U.S. Pat. No. 5,319,968 (Billing-Ross et al.) Here, an array of mirrors direct light rays through a ball lens. The shape of this lens is advantageous since beams which pass through the lens are at normal incidence to the image surface. The light rays are thus refracted toward the center of the lens, resulting in an imaging system having a wide field of view.
Another specialized use of ball lens characteristics is disclosed in U.S. Pat. No. 4,854,688 (Hayford et al.) In the optical arrangement of the Hayford et al. patent, directed to the transmission of a CRT-generated 2-dimensional image along a non-linear path, such as attached to headgear for a pilot, a ball lens directs a collimated input image, optically at infinity, for a pilot""s view.
Another use for wide-angle viewing capabilities of a ball lens is disclosed in U.S. Pat. No. 4,124,978 (Thompson), which teaches use of a ball lens as part of an objective lens in binocular optics for night viewing.
U.S. Pat. Nos. 4,124,978 and 4,854,688 described above that the use of a ball lens in image projection, there are suggestions of the overall capability of the ball lens to provide, in conjunction with support optics, wide field of view imaging. However, there are substantial problems that must be overcome in order to make effective use of such devices for immersive imaging applications, particularly where an image is electronically processed to be projected. For example, conventional electronic image presentation techniques, using devices such as spatial light modulators, provide an image on a flat surface. Ball lens performance with flat field imaging would be extremely poor.
There are also other basic optical limitations for immersion systems that must be addressed with any type of optical projection that provides a wide field of view. An important limitation is imposed by the LaGrange invariant. Any imaging system conforms to the LaGrange invariant, whereby the product of pupil size and semi-field angle is equal to the product of the image size and the numerical aperture and is an invariant for the optical system. This can be a limitation when using, as an image generator, a relatively small spatial light modulator or similar pixel array which can operate over a relatively small numerical aperture since the LaGrange value associated with the device is small. A monocentric imaging system, however, providing a large field of view with a large pupil size (that is, a large numerical aperture), inherently has a large LaGrange value. Thus, when this monocentric imaging system is used with a spatial light modulator having a small LaGrange value, either the field or the aperture of the imaging system, or both, will be underfilled due to such a mismatch of LaGrange values. For a detailed description of the LaGrange invariant, reference is made to Modern Optical Engineering, The Design of Optical Systems by Warren J. Smith, published by McGraw-Hill, Inc., pages 42-45.
Copending U.S. patent application Ser. Nos. 09/738,747 and 09/854,699 take advantage of capabilities for wide field of view projection using a ball lens in an autostereoscopic imaging system. In both of these copending applications, the source image that is provided to the projecting ball lens for each eye is presented as a complete two-dimensional image, presented on a surface. The image source disclosed in the preferred embodiment of each of these applications is a two-dimensional array, such as an LCD, a DMD, or similar device. The image source could alternately be a CRT which, even though generated by a scanned electron beam, presents a complete two-dimensional image to ball lens projection optics.
It can be appreciated by those skilled in the optical arts that a high brightness image source would be most advantageous for wide-field autostereoscopic imaging. However, in order to achieve suitable brightness levels for conventional autostereoscopic systems, LCD or DMD-based systems require complex and costly apparatus. CRT and OLED technologies, meanwhile, do not provide solutions that offer high brightness for wide-field autostereoscopic imaging. Thus, there is a recognized need for a simple, low cost, high-brightness image source that is well-suited to autostereoscopic imaging apparatus.
Resonant fiber optic scanning has been proposed for use in diagnostic instrumentation, such as in endoscopic equipment, for example. An article by Eric J Seibel, Quinn Y. J. Smithwick, Chris M. Brown, and Per G. Reinhall, entitled xe2x80x9cSingle fiber endoscope: general design for small size, high resolution, and wide field of viewxe2x80x9d in Proceedings of SPIE, Vol. 4158 (2001) pp. 29-39, describes the use of a vibrating, flexible optical fiber in 2-D scanning applications, where scanning is used for an input sensing function. When actuated at resonant frequency, a fiber optic element can be controllably scanned over an area to trace out a given regular pattern in a periodic fashion. Using this capability, U.S. Pat. No. 6,294,775 (Seibel et al.) discloses methods for controlled deflection of a flexible optical fiber as a scanning component in an image acquisition system.
While resonant fiber scanning is being employed for image acquisition functions, as noted in the above article and in U.S. Pat. No. 6,294,775, there may also be as yet unexploited advantages in using this technology for image formation, such as in image projection apparatus.
To take advantage of the benefits of resonant fiber scanning in an autostereoscopic display apparatus, copending U.S. patent application Ser. No. 10/095,341 discloses a two-dimensional image source that provides an intermediate image by scanning an optical fiber in a scan pattern synchronous with modulation of light emitted from the fiber. The apparatus disclosed in this copending U.S. Patent application employs a curved mirror for forming a virtual image of this intermediate image. This approach provides high brightness levels in a simple image generation system. However, there are advantages in alternative approaches that eliminate the cost of the curved mirror.
Thus it can be seen that, while there are some conventional approaches that meet some of the requirements for autostereoscopic imaging, there is room for improvement. There is particular interest in improved designs that maximize image brightness, minimize image aberrations, reduce constraints on viewer movement, and provide high-quality, high-resolution virtual images for pupil imaging with large viewing pupil sizes. In particular, solutions that reduce the number of components and minimize component cost and complexity are seen to be advantageous.
It is an object of the present invention to provide a substantially monocentric autostereoscopic optical apparatus for displaying a stereoscopic real image.
Briefly, according to one aspect of the present invention, a monocentric autostereoscopic viewing apparatus comprises a left image to be viewed by an observer at a left viewing pupil and a right image to be viewed by the observer at a right viewing pupil, the apparatus comprising:
(a) a left image generation system and, similarly constructed, a right image generation system, wherein each left and right image generation system forms a first intermediate curved image comprising an array of image pixels, with each image generation system comprising:
(a1) a light source for emitting modulated light corresponding to a series of image pixels arranged according to a scan pattern;
(a2) an optical waveguide having an input end coupled to the light source and a flexible output end for deflection, the output end emitting the modulated light;
(a3) an actuator for deflecting the flexible output end of the optical waveguide according to the scan pattern;
(a4) a curved surface for forming the first intermediate curved image thereon by receiving the modulated light emitted from the output end of the optical waveguide as deflected by the actuator according to the scan pattern;
(a5) an optical relay element for relaying, onto the curved surface, the modulated light emitted from the flexible output end of the optical waveguide according to the scan pattern, forming the first intermediate curved image thereby;
(b) a left ball lens assembly for projecting the first intermediate curved image from the left image generation system, the left ball lens assembly having a left ball lens pupil;
(c) a right ball lens assembly for projecting the first intermediate curved image from the right image generation system, the right ball lens assembly having a right ball lens pupil;
(d) a retro-reflective surface disposed to form, in cooperation with a beamsplitter, a pseudo-image of the left ball lens pupil at the left viewing pupil and to form a pseudo-image of the right ball lens pupil at the right viewing pupil; and the stereoscopic real image formed on the retro-reflective surface from the first intermediate curved image from the left image generation system and from the first intermediate curved image from the right image generation system.
A feature of the present invention is the use of a monocentric arrangement of optical components, thus simplifying design, minimizing aberrations and providing a wide field of view with large exit pupils.
A further feature of the present invention is the use of a resonant fiber optic image source for providing a scanned intermediate image.
A further feature of the present invention is that it allows a number of configurations, including configurations that minimize the number of optical components required, even including configurations that can function without a ball lens assembly.
It is an advantage of the present invention is that it eliminates the need for a higher cost two-dimensional surface as image source, replacing this with a lower cost scanned resonant fiber optic source.
It is a further advantage of the present invention that it allows use of inexpensive, bright light sources for generating an intermediate image for projection.
It is a further advantage of the present invention that it provides a compact arrangement of optical components, capable of being packaged in a display system having a small footprint.
It is a further advantage of the present invention that it allows high-resolution stereoscopic electronic imaging with high brightness and high contrast, with a very wide field of view. The present invention provides a system that is very light-efficient, capable of providing high brightness levels for projection.
It is a further advantage of the present invention that it provides a solution for wide field stereoscopic projection that is inexpensive when compared with the cost of conventional projection lens systems.
It is a further advantage of the present invention that it provides stereoscopic viewing without requiring an observer to wear goggles or other device.
It is yet a further advantage of the present invention that it provides an enlarged viewing pupil of sufficient size for non-critical alignment of an observer in relation to the display.
It is yet a further advantage of the present invention that it minimizes image aberration by projecting an image formed with an inherent curvature onto a curved surface.
It is yet a further advantage of the present invention that it provides uniform illumination across the full field of view.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.